Knowing: The Nature of Physical Law

  • Michael Munowitz
Oxford University Press: 2006. 432 pp £19.99, $35 0195167376 | ISBN: 0-195-16737-6

Among the many services that Michael Faraday rendered to science was his innovation of illustrating theoretical ideas by means of pictures and diagrams. For Faraday, who knew next to no mathematics but had a powerful visual imagination, the image was the concept. But as his successors dressed his ideas in the finery of nineteenth-century mathematics, we have come to think of mathematics as the essence of theory, with pictures being merely a helpful adjunct. Attempting to push the pendulum back a little, Michael Munowitz has written an account of modern physics that Faraday might appreciate, using no mathematics (well, all right, there is some arithmetic) but plenty of illustrations. His mixed success shows, rather sadly perhaps, how far theory has moved beyond our ability to capture it in an easily grasped visual form.

Munowitz plunges in with an opening chapter devoted to the four elementary forces of nature. Gravity holds the Solar System together, and electromagnetic forces do much the same thing for atoms. Going to a still smaller scale, we see how there must be a strong nuclear force to prevent positively charged protons from bursting out of the nucleus. This is nicely done. In each case, we perceive the force through its ability to control a physical system.

But then comes the pesky weak nuclear force, or rather interaction. The change of word betrays the problem. If there is some visualizable structure that the weak interaction holds together, I can't think of it, and neither can Munowitz. Instead, he talks about radioactive decays that the weak interaction engenders — and the connection to something that might be called a force is quietly dropped. I don't mean this as a criticism so much as a recognition of how difficult a task Munowitz has set himself.

Where Munowitz succeeds admirably, however, is in his recurrent emphasis on the way that principles of invariance and symmetry dictate the shape of natural law. Insistence that physics cannot depend on absolute velocity — there is no such thing — leads to the conservation law for momentum. Likewise, the impossibility of absolute time takes us (not so obviously) to the conservation of energy. In neither case can Munowitz make the connection precise, but he manages to make the reader feel that the conservation laws are not ex cathedra pronouncements but derive from eminently sensible assertions about the way the Universe is constructed.

A later chapter builds on these arguments to show how local gauge invariance of the electromagnetic field, for example, demands a force to make sure that particles behave as they must towards each other. And the tautological principle that an observer cannot tell the difference between identical quantum objects leads into a nice account of Pauli's principle and exchange forces. In these sections Munowitz splendidly demonstrates how a handful of bedrock principles underlie many apparently different areas of physics. The avoidance of mathematics is a real advantage here, forcing attention on to broad concepts rather than specific rules.

From time to time I had the nagging feeling that this book may be illuminating in the way that Richard Feynman's celebrated lectures are — by striking flashes of intellectual enlightenment in readers who have already learned physics the old-fashioned way. A young Faraday of our times, looking to this book for an education, would in the end be obliged to take many of the conclusions on trust. And in a few chapters, Munowitz fumbles the conceptual continuity, delivering catalogues of things that the dutiful reader ought to know. But for any high-school student or undergraduate who is losing sight of the forest on account of all the trees, Knowing at times offers a persuasively harmonious view of the physicist's way of looking at the world.